xwayland: Prevent kwin crashing when an xwayland socket cannot be created

If the X11 socket directory has wrong permissions, XwaylandSocket will
refuse to create sockets. Crashing in that case is undesired, just continue
executing without X.

src/xwayland/xwaylandlauncher.cpp

@@ -74,15 +74,16 @@ void XwaylandLauncher::enable()
     if (m_enabled) {
         return;
     }
-    m_enabled = true;
 
     if (!m_listenFds.isEmpty()) {
         Q_ASSERT(!m_displayName.isEmpty());
     } else {
-        m_socket = std::make_unique<XwaylandSocket>(XwaylandSocket::OperationMode::CloseFdsOnExec);
-        if (!m_socket->isValid()) {
-            qFatal("Failed to establish X11 socket");
+        auto socket = std::make_unique<XwaylandSocket>(XwaylandSocket::OperationMode::CloseFdsOnExec);
+        if (!socket->isValid()) {
+            qCWarning(KWIN_XWL) << "Failed to establish X11 socket";
+            return;
         }
+        m_socket = std::move(socket);
         m_displayName = m_socket->name();
         m_listenFds = m_socket->fileDescriptors();
     }
@@ -102,6 +103,8 @@ void XwaylandLauncher::enable()
             notifier->setEnabled(m_enabled);
         });
     }
+
+    m_enabled = true;
 }
 
 void XwaylandLauncher::disable()